Investigation and Determination Threshold and Rainy Season Length for Different Locations of Northeast of Iran (Khorasan Province)

Detection of rainfall characteristics in arid and semi-arid regions such as northeast of Iran has a critical role in any adaptation and mitigation plan to drought. The main goal of this study was to determine the rainy season starting date and daily rainfall threshold by using Rainfall Uncertainty Evaluation Model. The Starting Analysis Date (SAD) characterize the beginning of a new rainy season in a given region، whereas، the Rainy Season Beginning Date and the Rainy Season Ending Date are used to determine the Rainy Season Length. In this study، seventeen locations in northeast of Iran with available long daily rainfall data (24 to 48 years) were selected. Our results indicated three annual courses of rainy season، 12 locations showed a uni-model annual course with shorter values in summer (A courses)، 4 locations indicated uni-model with shorter rainy season length across April and March (B courses) and only 1 location showed bi-model annual course (C courses). These classifications were confirmed by multivariate statistical methods analysis. SAD was determined by differences between shortest median RSL and the shortest median RSL of the first day of each month. According to this approach SAD values for different classes determined as: July 1st in (A) region، March 1st in (B) region and February 1st in (C) region. Appropriate daily rainfall threshold was 1. 0 mm for only 3 locations، and the highest value of this index obtained in Torbat-j location (2. 2 mm)، therefore 1. 0 mm value which traditionally is in use as daily rainfall threshold need to be revised.